Recent work has highlighted that process–outcome relationships are likely to vary depending on the client, yet there is little direct evidence regarding specific intervention effects in individual clients. This study attempted to address the hypothesis that some clients reveal more than others regarding the impact of specific interventions. Intensive case study analyses were applied to 2 clients with principal major depressive disorder and comorbid anxiety disorders receiving transdiagnostic psychotherapy. Clients completed a battery of symptom and psychological assessments of mindfulness, cognitive reappraisal use, and emotion avoidance on many occasions throughout treatment. Time series analyses were applied to symptom and change construct data. Results included: (a) significant decreases in depression, anxiety, and emotion avoidance from baseline to posttreatment were observed, as well as significant increases in mindfulness and reappraisal; and (b) in one case, intervention strategies exerted little influence on changes in key variables; in the other, emotion exposure strategies had the strongest influence on increases in mindfulness and present-focused awareness strategies had the strongest influence on reductions in emotion avoidance. Even when different clients appear to similarly benefit from the same treatment, specific intervention effects on putative change factors may be more prevalent for some clients and less prevalent for others. Regular assessment is needed to determine if a client requires an alternative set of specific intervention strategies.